This invention relates to methods and apparatus for determining the disposition of earth strata traversed by a bore hole and concerns improved methods and apparatus using acoustic pulse energy transmission in fluid filled bore holes for detecting the angle and azimuthal direction of the dip of earth formations.
It is conventional practice to use a dip meter for determining the angle and azimuthal direction of the dip or the inclination of the earth formation strata traversed by a bore hole. One common form of dip meter makes simultaneous resistivity measurements at three or four equally spaced electrodes in a plane perpendicular to the axis of the bore hole. The electrodes provide a log of the resistivity of the surrounding formation as the electrodes traverse the bore hole wall. The logs from all of the resistivity electrodes are correlated in order to derive the relative displacement along the bore hole of the points of intersection with the plane of the formation geological dip. Inertial sensors with the dip meter provide additional information relating to the direction and slant of the bore hole and the rotational attitude of the tool itself.
Resistivity dip meters, while effective in bore holes without fluids or in water filled bore holes, have not been particularly effective in bore holes containing oil-based drilling mud. This is primarily due to the insulating quality of the oil-base drilling fluid since effective resistivity logging depends on the conductivity or resistivity measurement obtained through the drilling fluid and the insulating layer of mud cake adhering to bore hole wall. To overcome this problem to a degree special "scratcher" and "poller" electrodes have been employed to scrape through the insulating layer of mud cake and then make contact with the formation in order to make the resistivity measurement. Such attempts have the objective to optimize the electrical contact of the electrode with the bore hole wall. However, these attempts have met with varying degrees of success, often producing intermittent dip information.
Attempts have been made to use acoustical energy in obtaining information in order to determine formation dip measurements. The need is not for an absolute measurement but for a well log which can determine the change or transition from one earth stratum or formation to another. Attempts have been made to transmit acoustic pulse energy into the formation from a transducer in contact with the bore hole wall and then to measure the transition time of the acoustic energy through the formation to spaced receiving transducers also in contact with the bore hole wall. Other attempts have been made to direct the acoustic pulse energy at the bore hole wall/formation interface and to measure the acoustic pulse energy reflected therefrom, which would be indicative of the acoustic impedance of the formations. In both cases above described, however, it is not possible to place the acoustic transmitting transducer in continuous direct contact with the bore hole wall because of the random wall geometry that occurs during drilling, and any invasion of the oil-base drilling fluid between the transducers and the bore hole wall/formation interface will introduce some attenuation in the acoustic pulse energy. It has been found the heavier the "weight" of the oil-base mud, the greater the attenuation of the acoustic energy, due to scattering produced by the "weighting" material, such as barite or hematite, and the viscosity of the oil-base mud. In addition, in a pulse-echo configuration, the "noise" generated by the ringing decay of the transmitting transducer complicates the detection and accurate determination of the reflected acoustic pulse energy. If a "delay line" spacing is built into the transducer design between the transducer crystal and the bore hole fluid, such as a "delay line" to delay the transducer ringing, the delay line also introduces additional reflection and transmission losses. Further, in the first case described above, the acoustic pulse "travel" through the formation itself to the receiving transducers introduces additional attenuation. In many cases acoustic transducers are intended to substantially contact the bore hole wall and to remain in static position for respective transducer pulses. U.S. Pat. No. 5,146,050 of Strozeski, et al. evidences this general type of acoustic formation dip logging instrument. In other cases, as evidenced by U.S. Pat. No. 5,001,676 of Broding, an acoustic transducer is rotatably mounted within a transducer housing and is typically rotated 360.degree. by a motor and shaft at a rotational velocity of from 100 to 400 rpm while the transducer is pulsed in the range of about 2000 cycles per second. Utilization of a rotary acoustic transducer of this nature requires a complex transducer design which makes it both expensive to manufacture and troublesome to use. Accordingly, it is desirable to provide a bore hole logging instrument having a high pulse rate and yet having a simplified and trouble free transducer design.